After a geothermal power generation system generates power by using steam or hot water blown out of a production well, the hot water—the temperature of which has dropped—is made to return to an injection well. Since the hot water of high temperature blown out of the production well contains more calcium and dissolved silica than those in the well water or the river water, scale such as calcium carbonate or amorphous silica is easily precipitated. In particular, in the terrestrial part and in the injection well, there is a problem of suppressing generation of silica scale due to the temperature drops of hot water in the terrestrial part.
Generally, a sulfuric acid injection method is used as a suppression method of the silica scale. In the sulfuric acid injection method, the polymerization rate of silica is suppressed by lowering the pH of hot water to reduce the scale precipitation amount. However, as the sulfuric acid injection method merely decreases the polymerization rate of silica, silica is expected to be precipitated after a sufficient time passes in the injection well. In addition, as the sulfate ion density increases, the possibility that scale such as anhydrite or the like is precipitated increases. Further, there is a problem that piping or the like is eroded with acid.
As an attempt to solve these problems, there is a method of alkalizing hot water (for example, see Daisuke Fukuda, Geothermal Technology, Vol. 34, Nos. 1 & 2 (Ser. No 74) 51-57, 2009) (hereinafter “Fukuda”). In other words, the solubility of amorphous silica becomes higher as the alkalinity become higher, and it suddenly rises at pH 8 or more, in particular. Therefore, silica scale is hardly generated in a high pH solution. Further, this effect continues in the injection well, since the silica precipitation amount does not increase even if the time passes, which contrasts with the above-mentioned method of suppressing the rate of silica polymerization. Furthermore, disclosed is a method of suppressing the precipitation of calcium carbonate, anhydrite, or magnesium silicate in the production well, by also using a chelating agent of catching calcium or magnesium in the production well.